[Rd] "[", .local and S4 methods (was: "[" operator and indexing ambiguity)

[Wolfgang Huber]

Dear Tony,

thanks for the tip with "nargs", when suitably applied, this answers the 
problem.

The behaviour of "nargs" in S4 methods has some subtleties compared to 
that in normal functions, as shown in the example below. I admit that 
this is what had earlier created some confusion about the semantics of 
"nargs". From the perspective of "nargs" and its man page, this all 
seems fine, but its interaction with the S4 methods implementation was 
surprising to me and I'd be interested whether people think that this is 
the desired behaviour.

### ------------------- example code ---------------------------

setClass("foo", representation(x="array"))
a = new("foo")

myfun = function (x, i, j, k, ..., drop=TRUE) {
   cat(paste(match.call()), "\nnargs()=", nargs(),
       missing(i), missing(j), missing(k), missing(drop), "\n\n")
   return(invisible(NULL))
}
setMethod ("[", signature(x="foo", i="numeric", j="missing"), myfun)

a[7]
a[7,]
myfun(a, 7)
myfun(a, 7, )
showMethods("[", classes="foo", inc=TRUE)

### --------------------- example output -----------------------

.local x i j drop
nargs()= 4 FALSE TRUE TRUE FALSE

.local x i j drop
nargs()= 4 FALSE TRUE TRUE FALSE
    ## here it seems impossible to distinguish a[7] from a[7,]
    ## also note that missing(drop) is FALSE

myfun a 7
nargs()= 2 FALSE TRUE TRUE TRUE

myfun a 7
nargs()= 3 FALSE TRUE TRUE TRUE

Function: [ (package base)
x="foo", i="numeric", j="missing"
function (x, i, j, ..., drop = TRUE)
{
     .local <- function (x, i, j, k, ..., drop = TRUE)
     {
         cat(paste(match.call()), "\nnargs()=", nargs(), missing(i),
             missing(j), missing(k), missing(drop), "\n\n")
         return(invisible(NULL))
     }
     .local(x, i, j, ..., drop = drop)
}



### ----------- example code 2 (different signature: no k) ---

myfun = function (x, i, j, ..., drop=TRUE) {
   cat(paste(match.call()), "\nnargs()=", nargs(),
       missing(i), missing(j), missing(drop), "\n\n")
   return(invisible(NULL))
}
setMethod ("[", signature(x="foo", i="numeric", j="missing"), myfun)


a[7]
a[7,]
myfun(a, 7)
myfun(a, 7, )
showMethods("[", classes="foo", inc=TRUE)

### --------------------- example output -----------------------


[ a 7
nargs()= 2 FALSE TRUE TRUE

[ a 7
nargs()= 3 FALSE TRUE TRUE

myfun a 7
nargs()= 2 FALSE TRUE TRUE

myfun a 7
nargs()= 3 FALSE TRUE TRUE

Function: [ (package base)
x="foo", i="numeric", j="missing"
function (x, i, j, ..., drop = TRUE)
{
     cat(paste(match.call()), "\nnargs()=", nargs(), missing(i),
         missing(j), missing(drop), "\n\n")
     return(invisible(NULL))
}

########################################

 > sessionInfo()

R version 2.5.0 Under development (unstable) (2007-01-28 r40602)
i686-pc-linux-gnu

locale:
LC_CTYPE=en_GB.UTF-8;LC_NUMERIC=C;LC_TIME=en_GB.UTF-8;LC_COLLATE=en_GB.UTF-8;LC_MONETARY=en_GB.UTF-8;LC_MESSAGES=en_GB.UTF-8;LC_PAPER=en_GB.UTF-8;LC_NAME=C;LC_ADDRESS=C;LC_TELEPHONE=C;LC_MEASUREMENT=en_GB.UTF-8;LC_IDENTIFICATION=C

attached base packages:
[1] "stats"     "graphics"  "grDevices" "utils"     "datasets"  "methods"
[7] "base"

Best wishes
-- 
------------------------------------------------------------------
Wolfgang Huber  EBI/EMBL  Cambridge UK  http://www.ebi.ac.uk/huber